1. Field of the Invention
The present invention relates to polyester polyols and their use, in combination with modified polyisocyanates, as the binder in two-component polyurethane coating compositions for plastic moldings which provide excellent resistance to yellowing and tar stains.
2. Description of the Prior Art
It is known to coat substrates of low flexibility, e.g., metal or wood, with two-component polyurethane coating compositions based on polyesters, polyethers or polyacrylates containing hydroxyl groups, and organic polyisocyanates. The resulting coatings are distinguished in particular by excellent hardness, very good adhesion and high weather resistance. The chemical components of these lacquers and coatings are described inter alia in "Lackkunstharze" ("Synthetic Resins for Lacquers"). Hans Wagner/Hans Friedrich Sarx, Carl Hanser Verlag, Munich, pages 153 to 173, 1971.
However, these known two-component polyurethane coating compositions often result in highly crosslinked coatings whose elasticity and tendency to yellow, especially on irradiation with short-wave UV light, frequently do not satisfy the corresponding requirements of coatings for flexible substrates.
Flexible plastic parts are being used to an increasing extent for improving safety, especially in automobile construction. Thus, in some countries motor vehicles have to be equipped with bumpers which can withstand a collision at a particular speed (e.g., 5 mph) against a solid barrier without being damaged, while at the same time functional parts, such as headlamps, direction indicators, doors and bonnets, must not have their function impaired. In many countries, these and other even more extensive demands have led to the replacement of metal bumpers on automobiles with complete front and rear parts made of highly flexible materials which satisfy the demands on the ability of the bumper to recover without damage after such low impact collisions.
Highly flexible plastics which have proved suitable for this application include semirigid elastomeric polyurethanes (e.g., the Bayflex systems from Bayer AG, Leverkusen), which are manufactured from two-component polyurethane forming mixtures by the reaction injection moulding technique in closed molds, optionally with foaming; thermoplastic polyurethanes (e.g. the Desmophan elastomers from Bayer AG or the Texin elastomers from Miles Inc.) processed by the injection molding; and various grades of rubber.
Plastic parts based on these chemical materials, which have been used in vehicle construction for some years, are relatively large and therefore have a substantial affect on the appearance of the vehicle, which is why it is necessary to coat these parts. In addition, the surfaces of the plastics are degraded upon exposure to the weather and therefore have to be protected from the effects of the weather.
Flexible coatings are also required for less flexible plastic parts in order to protect the parts from mechanical damage. Thus, rigid but tough thermoplastics have to be coated with highly flexible, extremely resistant coatings in order to prevent the lacquer film from cracking due to mechanical damage or other external influences, and to prevent the cracks from propagating into the compact plastic.
It is possible using the known two-component compositions, to prepare highly flexible coatings which are totally adequate with respect to elasticity and low-temperature flexibility. This is made possible especially by using, as the essential polyol component, particular polyhydroxy polyesters which are synthesized predominantly from aliphatic diols and at most have a hydroxyl functionality slightly greater than 2. However, the disadvantage of two-component polyurethane coating compositions based on these polyester diols and the known lacquer polyisocyanates is the fact that the coatings have inadequate weather resistance and chalk resistance, such that the gloss retention of the coatings built up from such coating compositions is totally unsatisfactory. The susceptibility of these coating systems to yellowing on irradiation with short-wave UV light is particularly disadvantageous. Another serious disadvantage is the lack of resistance of the coatings to tar stains. This is especially true with regard to the systems of DE-OS 3,421,122, which otherwise substantially meet the requirements of flexible plastic coatings.
A significant improvement in the susceptibility of highly flexible polyurethane coating compositions to yellowing on irradiation with short-wave UV light can be achieved with the polyesterpolyols described in EPA 0,318,800. Furthermore, these polyester polyols also afford improvements in the resistance of corresponding polyurethane coating compositions to tar stains, although these improvements are inadequate in terms of current requirements.
An object of the present invention is to provide a two-component polyurethane coating composition which satisfies these special requirements and is especially suitable for coating flexible plastic parts. In particular, the coating composition should satisfy all of the following requirements:
1. Non-yellowing coatings upon exposure to shortwave UV light: PA0 2. Low-temperature elasticity: PA0 3. Resistance to tar stains: PA0 4. Good gloss retention: PA0 5. No post-embrittlement on exposure to the weather: PA0 6. Low drying temperature: PA0 7. Repairability: PA0 a) 52 to 60 mole % of a polyol component containing PA0 b) 40 to 48 mole % of a dicarboxylic acid component containing
The coating must resist irradiation from shortwave UV light (wavelength &lt;400 nm) over a period of at least 200 h without yellowing. PA1 Even at -20.degree. C., the film must not crack during low impact collisions and the mechanical properties of the plastic should be impaired as little as possible by the coating. PA1 The coating must be able to resist the action of a special tar-containing test solution. PA1 The coating on the plastic parts should be as durable as possible, i.e., under the effects of the weather, it must not degrade more rapidly than the coatings on metal parts. PA1 Even after prolonged exposure to the weather, the film must not suffer from post-embrittlement and the low temperature elasticity should not deteriorate during exposure to the weather. PA1 Because the heat deflection temperature of such large plastic parts is limited, the requisite drying temperatures and times of the coating compositions should therefore be minimized for this reason and also for energy saving reasons. PA1 The drying conditions of the coating composition should not only be sufficient for initial application, but also for repair coating at 80.degree. C. or room temperature. PA1 i) 10 to 50 mole % of one or more dihydric aliphatic alcohols having at least two carbon atoms, other than neopentyl glycol, PA1 ii) 5 to 40 mole % of one or more trihydric or higher functional aliphatic alcohols having at least 3 carbon atoms, PA1 iii) 10 to 40 mole % of one or more cycloaliphatic diols and PA1 iv) 10 to 60 mole % of neopentyl glycol, with PA1 v) 0 to 49.9 mole % of one or more saturated aliphatic dicarboxylic acids or anhydrides thereof having at least 2 carbon atoms and PA1 vi) 50.1 to 100 mole % of one or more unsaturated aliphatic dicarboxylic acids or anhydrides thereof having at least 4 carbon atoms,
It has now been surprisingly found that these objectives may be achieved, in particular the resistance of the coatings to tar stains, by using the polyester polyols according to the invention as the polyhydroxyl component or a substantial portion of the polyhydroxyl component in two-component polyurethane coating compositions.